I stumbled through several different upgrade paths this year, but settled on:
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1. Filter paper to germinate.
2. 1 sproutling per 8 oz coffee cup.
3. 6+ gallon wallybags.
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Earlier, I tended to add a 4~5" pot stage (If nothing else, because it's so much more convenient to lug them around!), but found the plants were often becoming slightly root-bound sooner than anticipated. Further, I noticed that the plants whose roots roots hit the edge of the pot and started to go 'sideways' seemed to lag non-root-bound plants by several weeks. My current philosophy is to give them as much space as possible. "Just Grow, Baby!"